More than sunshine

A plain guide to vitamin D: where it comes from, which form to choose, and how to take it.

Vitamin D has the friendliest nickname in the supplement aisle: the sunshine vitamin. It's a fair name, since your skin makes vitamin D when sunlight hits it. That's also where the common assumption comes from: if you spend time outdoors, you're covered.

The sun does a lot of the work, but how much depends on more than stepping outside. The season, the time of day, the clouds, your sunscreen, your skin tone, and your age all change how much vitamin D your skin makes.¹ And very few foods contain much of it to make up the difference.¹

So here's how vitamin D works, where it comes from, what to look for on the label, and how to take it so more of it reaches you.

The Calcium Connection

Vitamin D's main job is helping you absorb calcium. Calcium is the mineral your bones are built from, but eating it is only half the story. Vitamin D is what helps your gut take it in.¹

That makes vitamin D part of how your bones stay strong. Your skeleton isn't fixed. It's constantly being broken down and rebuilt, and vitamin D is involved in that process.¹

Its reach goes beyond bones, too. Vitamin D plays a part in how your muscles and nerves work together, and in how your immune system functions.¹

D2 vs D3

Since so few foods contain much vitamin D, food alone rarely covers it. Fatty fish is one of the few natural sources, and in the US, fortified foods supply most of the vitamin D people eat.¹ That's why so many people reach for a supplement.

Pick up a bottle, and the label will name one of two forms. They look almost identical on paper, but they behave differently once they're in you.

Vitamin D3 (cholecalciferol)

The form your own skin makes when sunlight hits it.¹
Found in small amounts in animal foods like egg yolks, cheese, and beef liver.¹
In supplements, usually made from lanolin, a substance that comes from sheep's wool.¹
In a study where people took one large dose, blood levels of vitamin D kept climbing for two weeks afterward.²

Vitamin D2 (ergocalciferol)

Made by exposing yeast to UV light, and the kind mushrooms provide, in amounts that vary.¹
In the same study, it was absorbed just as well as D3, and blood levels rose about the same over the first few days.²
Then levels dropped quickly and were back where they started by day 14. Overall, it had less than a third of D3's effect.²

A review of trials that compared the two head to head reached the same verdict: D3 did a better job of raising vitamin D levels overall.¹·³ So when you're reading labels, D3 is the one to look for.

Why D3 Comes With K2

Look at many vitamin D supplements and you'll see K2 listed alongside it. The pairing isn't random.

Both are fat-soluble vitamins, and both play a central role in how your body handles calcium.⁴ Vitamin D tells your body to make certain proteins that work with calcium. But those proteins can't do their job until vitamin K switches them on.⁴ One makes the tools, and the other gets them working.

One of the proteins vitamin K activates, called osteocalcin, sits in your bones.⁵ Most of the research on the pair has looked at bone health, and researchers reviewing the evidence suggest the two may work better together than either one alone.⁴

Vitamin K itself comes in two main forms. K1 is the kind in leafy greens, and it's the main form people get from food. K2 comes mostly from fermented and animal foods, in more modest amounts.⁵ On a supplement label, K2 usually appears as MK-4 or MK-7.⁵

How Much You Need and How to Take It

Here are the official daily targets for adults:

Vitamin D: 600 IU (15 mcg) a day up to age 70, and 800 IU (20 mcg) after that.¹
Vitamin K: 90 mcg a day for women and 120 mcg for men.⁵

Vitamin D also has an upper limit: 4,000 IU (100 mcg) a day for adults.¹ Past that point, more isn't better. If you want to know whether you need more than the basics, a blood test will tell you. The one doctors use measures 25-hydroxyvitamin D, often written as 25(OH)D.¹

Vitamin K is different. No upper limit has been set for it, because it has a low potential for toxicity.⁵

A couple of things help it all work:

Take it with a meal that has some fat in it. Both vitamins are fat-soluble,⁴ and fat in your gut helps vitamin D get absorbed.¹ In one study, vitamin D3 taken with a meal containing fat reached about a third higher levels in the blood than when taken with a fat-free meal. The kind of fat made no difference.⁶
If you take prescription medication, check with your doctor first. People on blood thinners in particular need to keep their vitamin K intake steady.⁵

Sun, Plate, or Pill

Vitamin D lives up to its nickname, but the sun can't always do the job on its own.

Your plate can help too. Fatty fish are among the best natural sources: a 3-ounce serving of cooked salmon has about 570 IU, close to a full day's target for most adults.¹ Trout is in the same range, and mushrooms exposed to UV light, eggs, and fortified milk add smaller amounts.¹

When sunlight and food still fall short, a supplement is a simple way to fill the gap, and it's worth choosing one with a little care.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

References

Office of Dietary Supplements, National Institutes of Health. Vitamin D — Fact Sheet for Health Professionals. ods.od.nih.gov.
Armas LA, Hollis BW, Heaney RP. Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab. 2004;89(11):5387-5391. doi:10.1210/jc.2004-0360.
Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. doi:10.3945/ajcn.111.031070.
van Ballegooijen AJ, Pilz S, Tomaschitz A, Grübler MR, Verheyen N. The synergistic interplay between vitamins D and K for bone and cardiovascular health: a narrative review. Int J Endocrinol. 2017;2017:7454376. doi:10.1155/2017/7454376.
Office of Dietary Supplements, National Institutes of Health. Vitamin K — Fact Sheet for Health Professionals. ods.od.nih.gov.
Dawson-Hughes B, Harris SS, Lichtenstein AH, et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. doi:10.1016/j.jand.2014.09.014.

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